1. Technical Field
This invention relates generally to a differential to single-ended conversion scheme and, more particularly, to a circuit for converting a differential voltage produced from a magnetic or variable reluctance sensor to a single-ended voltage that is used with an adaptive threshold circuit.
2. Discussion
Inductive magnetic sensors are commonly employed for automotive applications and the like to provide timing signals which enable the determination of position and speed of a rotating wheel. For example, specific applications may include the determination of engine crankshaft position and speed (i.e., RPM) or the determination of wheel speed for anti-lock braking systems. Inductive magnetic sensors used for these types of applications are commonly referred to as variable reluctance sensors.
The variable reluctance sensor is generally located adjacent to a rotating wheel which typically has a plurality of circumferentially spaced slots formed therein. The sensor has an inductive magnetic pick-up that is generally made up of a pick-up coil wound on a permanent core. As the wheel rotates relative to the pick-up coil, an alternating voltage is generated in the pick-up coil when the slots on the wheel travel past the sensor. The frequency of the alternating voltage is then determined to achieve rotational speed information about the wheel.
The alternating voltage that is produced with the variable reluctance sensor has peak voltages that generally vary in amplitude according to the rotational speed of the wheel. In a number of automotive applications, the amplitude may vary from 250 millivolts (mV) at low end speeds to upwards of 200 volts peak-to-peak at higher rotating speeds. The sensor output voltage is usually processed by a threshold circuit and translated to a digital pulse train of say 0-5 volts which in turn provides the frequency information to a control or processing module.
Variable reluctance sensors are commonly subjected to potential errors caused by noisy environments associated therewith. For instance, noise may be injected into sensor wires which are located in close proximity to high voltage ignition coils. Also, dirt or scratches existing on the surface of the wheel may cause noise. The amount of noise generally increases in proportion to the speed of the wheel. To eliminate some of these noise problems, a number of adaptive threshold schemes have been developed to decode the alternating waveform. However, adaptive threshold schemes are generally configured to receive a single-ended voltage. In the past, the single-ended voltage was commonly achieved by connecting one end of the variable reluctance sensor to ground. However, such grounded single-ended sensors remain highly sensitive to high voltage noise which may be injected into the signal through the ground connection or otherwise.
For applications which are susceptible to be affected by large amounts of noise, it is desirable to employ a differential voltage output from a variable reluctance sensor. This is accomplished by connecting both ends of the pick-up coil to separate output lines. However, the differential voltage may have to be converted to a single-ended voltage which may be used with a threshold circuit.
One object of the present invention is to provide a circuit that is capable of converting a differential voltage that is generated from a variable reluctance sensor to a single-ended voltage.
Another object of this invention is to provide a differential to single-ended conversion circuit that does not suffer from the sensitivities of noise injection through a ground connection.
Still another more specific object of this invention is to provide for such a differential to single-ended conversion circuit that allows for a differential voltage to be used with low noise interference, and which may then be converted to a single-ended voltage for use with a threshold circuit.